Stochastic Threshold for Ion Heating with Beating Electrostatic Waves

Abstract

The stochastic threshold for the heating of ions in a magnetized plasma with two electrostatic waves is experimentally characterized. Two obliquely propagating electrostatic modes are launched in a magnetized plasma with frequencies that differ by the ion cyclotron frequency. The values of the wave amplitudes where a rapid increase in the local ion temperature occurs is then parametrically investigated. It is found that the two threshold wave amplitudes are linearly related and that this dependence translates to a lower required energy density for the onset of heating when compared to the case of a single electrostatic wave. Agreement also is demonstrated between the experimentally observed threshold for stochastic heating and an analytical prediction [B. Jorns and E. Y. Choueiri, Phys. Rev. E 87, 013107 (2013)] for this threshold.

Experimental Characterization of Plasma Heating with Beating Electrostatic Waves

“…18 In this low amplitude case, the BEWH effect is in fact superior-another important consideration for experiments with extremely limited available power.…”

“…This suggests the onset of a stochastic transition, the origin and study of which were the subject of a number of investigations. 17,18 It is this stochastic transition that facilitates the energy exchange of the waves with the plasma. In the second region, there is a nonlinear increase in ion temperature with antenna current.…”

“…18 where these threshold effects were examined). In this case of large amplitude, particle orbits are sufficiently de-correlated to permit a random phase approximation such that the power absorption for the BEW process is given by (c.f.…”

“…13,14,16 One of the more important results that stemmed from that work is the demonstration that the onset of heating occurs for BEW at lower total energy density than for the case of SEW heating. 17,18 The combination of kinetic formulation and single particle analysis thus have provided a full description of when heating occurs with BEW and how the power is deposited in the plasma ions. In the context of our motivation to more efficiently heat plasmas, these tools permit us to pose the central question: is BEW heating superior to SEW heating?…”